Electrochemical devices such as battery cells comprise an electrolyte layer where ion transfer takes place and an electrode layer where electron transfers to ions takes place together with the ion transfer. Polymer compounds are added to the electrolyte layer and electrode layer for the following purposes.
1) Addition to the Electrolyte Layer
Since electrolytes are usually liquid with a supporting salt dissolved in a solvent, and hence require a container for containing the liquid, electrochemical devices using such electrolytes are difficult to make smaller and thinner. To solve this problem, researches are being conducted on all-solid state electrochemical devices using solid electrolytes in place of traditional liquid electrolytes.
Among others, lithium batteries have been researched vigorously as the type of battery that can obtain high energy density since lithium is a substance with a light atomic weight and large ionization energy, and nowadays, lithium batteries are extensively used as power sources for portable appliances.
On the other hand, with the widespread use of lithium batteries, concern has been growing in recent years about the safety of batteries because of increased internal energy associated with the increase in the amount of active material content and also because of increasing amounts of organic solvents which are flammable materials used as electrolytes.
As a method to ensure the safety of lithium batteries, it is extremely effective to use solid electrolytes, which are nonflammable, in place of organic solvent electrolytes. It is therefore important to use solid electrolytes for lithium batteries in order to ensure high safety levels as well as to achieve the earlier noted small and thin construction.
Materials such as lithium halide, lithium nitride, lithium oxygen acid salt, or their derivatives, are known as materials for lithium ion conductive solid electrolytes used in such batteries. Amorphous solid electrolytes of lithium ion conductive sulfides such as Li.sub.2 S-SiS.sub.2, Li.sub.2 S-P.sub.2 S.sub.5, Li.sub.2 S-B.sub.2 S.sub.3, and the like, and lithium ion conductive solid electrolytes formed from such glasses doped with a lithium halide such as LiI or a lithium salt such as Li.sub.3 PO.sub.4, are known to exhibit high ionic conductivity of the order of 10.sup.-4 to 10.sup.-3 S/cm or higher.
As compared with these inorganic solid electrolytes, a polymer solid electrolyte comprising an organic substance is obtained from a solution of a lithium salt and an organic polymer compound by allowing the solvent to evaporate. This polymer solid electrolyte has excellent workability compared with inorganic solid electrolytes, in that it can be easily formed into a thin film and in that the resulting solid electrolyte thin film has flexibility.
As a solid electrolyte having flexibility or rubber elasticity, there has recently been proposed a novel solid electrolyte, named the "polymer in salts" electrolyte, that comprises an inorganic salt and a polymer and has lithium ion conductivity of extremely high density compared with the above-described polymer solid electrolyte (C. A. Angell, C. Liu, and E. Sanchez, Nature, vol. 362, (1993) 137).
In electrochemical devices using liquid electrolytes also, a porous polymer compound is usually used as a separator in the electrolyte layer. The separator may mechanically prevent an electrical contact between the electrodes, and is required not only to have excellent liquid retentivity for retaining the liquid electrolyte and be chemically stable in the electrochemical device, but also to be electrochemically stable since it is used in contact with the electrodes.
2) Addition to the Electrode Layer
An electrode is formed by molding an electrode active material and contacting the same with a current collector. If the electrode active material is simply molded by a pressure molding process, the cohesive force working between electrode active material particles primarily depends only on van der Waals forces. However, since conventional electrochemical devices use liquid as the electrolyte, if the molded electrode formed by the pressure molding process alone is immersed in the liquid electrolyte, liquid molecules are adsorbed onto the surfaces of the electrode active material particles, as a result of which the cohesive force working between the active material particles decreases and active material particles drop off the molded electrode into the liquid electrolyte, resulting in that the shape of the molded electrode cannot be retained. To increase the formability of the electrode, usually a polymer compound is added as a binding agent to the molded electrode.
To the electrolyte layer or electrode layer of an electrochemical device, polymer compounds are added for the above-described purposes, but the prior art techniques have had the following problems.
The inorganic solid electrolytes described above are ceramic or glass, and in battery cell applications, the materials are usually used in the form of pellets obtained by pressure molding pulverized solid electrolyte powder. However, since the pellets are hard and brittle, there has been the problem that they lack workability and are difficult to be made thin.
The organic solid electrolytes, on the other hand, have low ionic conductivity of the order of 10.sup.-4 S/cm or less at room temperature, which has not been sufficient for practical lithium cell electrolytes. To solve this problem, it has been proposed to make a polymer solid electrolyte with increased ionic conductivity by adding a plasticizer. However, plasticizers are flammable by their nature, and the addition of a plasticizer in turn gives rise to such problems as decreased lithium ion transport number or decreased reactivity with the lithium anode. Furthermore, whether a plasticizer is added or not, it is hard to say that these organic solid electrolytes have sufficient performance as lithium battery electrolytes.
Further, most of the solid electrolytes generally known as the "polymer in salt" electrolyte have low ionic conductivity, of the order of 10.sup.-4 S/cm or less, which cannot be said to be sufficient for lithium battery electrolytes. If an ambient temperature molten salt such as AlCl.sub.3 -LiBr-LiClO.sub.4 is used as the inorganic salt, high ionic conductivity can be obtained, but this in turn tends to cause an electrochemical reduction of aluminum and, therefore, cannot be said to be suitable for lithium cell electrolytes.
As earlier described, the molded electrode is constituted by a mixture, which is prepared by mixing a polymer compound as a binder into the electrode active material. The polymer compound is usually an electrically insulating substance and tends to interfere with the ion transfer, thus interfering with the electrochemical reaction occurring at the electrode/electrolyte interface and also the dispersion of ions within the electrode. If the mixing ratio of the polymer compound is increased to improve the formability, there arises the problem that the operating characteristics of the electrochemical device tend to drop.
Further, the molded electrode is formed by mixing in a dispersing medium a mixture comprising an electrode active material, a binder, and an electron conductive material added if necessary to increase the electron conductivity within the electrode, and by loading or coating a current collector with the resulting slurry and allowing the dispersing medium to evaporate. To enhance the coating or loading properties of the slurry, it is desirable that the polymer compound used as the binder be soluble in the dispersing medium used.
When a solid electrolyte is used as the electrolyte, particles of the electrode active material are prevented from being separated and dropping into the electrolyte. However, in that case also, if the molded electrode is formed by simply pressure molding an electrode active material, or a mixture of an electrode active material and a solid electrolyte to increase the reacting surface area, the molded electrode is hard and brittle and lacks workability, the resulting problem being difficulty in constructing the electrochemical device.
Further, when a solid electrolyte is used as the electrolyte, since the contact interface with the electrode active material is a solid/solid interface, the contact surface area between the electrode active material and the electrolyte becomes smaller than when a liquid electrolyte is used. This therefore, tends to increase the electrode reaction resistance. When an electrically insulating polymer compound is added to improve the formability, this tendency becomes more pronounced. This has therefore led to the problem that the electrode reaction speed tends to drop.
Taking the lithium battery as an example of the electrochemical device, lithiated cobalt oxide (Li.sub.x CoO.sub.2) or the like is used as the cathode active material, and graphite or the like as the anode active material. Since these materials are obtained as powder, if they are simply pressure molded into molded electrodes for use in the lithium battery, as earlier described, the liquid electrolyte penetrates between the electrode constituent particles, causing the electrodes to swell and thus resulting in the problem that not only does it become difficult to retain the shape but the electrical contact also tends to be lost.
Further, Li.sub.x CoO.sub.2 has a structure of a triangular lattice of oxygen, lithium, and cobalt stacked in the order of O, Li, O, Co, O, Li, and O, and lithium ions are accommodated between the CoO.sub.2 layers. Through an electrochemical oxidation reduction reaction within the lithium ion conductive electrolyte, the lithium ions move in and out the space between the CoO.sub.2 layers. As a result, the degree of electrical interaction between the CoO.sub.2 layers varies, causing the layer spacing to expand and contract and hence a volumetric change of the electrode. This has lead to the problem that as charge/discharge cycles are repeated, bonds between the electrode forming particles tend to be lost and the capacity drops with charge/discharge cycles.
The above description has been given by taking Li.sub.x CoO.sub.2 as an example of the electrode active material; materials traditionally used as the lithium cell active materials or materials expected to be used in the future include transition metal oxides such as Li.sub.x NiO.sub.2, Li.sub.x MnO.sub.2, MnO.sub.2 and the like, transition metal disulfides such as Li.sub.x TiS.sub.2 and the like, graphite intercalation compounds, and graphite fluorides. Similar problems can arise with these materials.
Further, when a solid electrolyte is used as the electrolyte, the contact area between the solid electrolyte and the electrode active material tends to decrease, as earlier noted. Accordingly, when a volumetric change occurs in the electrode active material in association with the charge and discharge operations of the cell, bonds between the active material and the electrolyte tend to be lost. Moreover, since cell materials are all formed from solid substances, and there are no elastic members for absorbing the volumetric change of the electrode active material during charging and discharging, a dimensional change may occur in the cell, leading to sealing failure of cell seals.
An object of the present invention is to provide a molded solid electrolyte that solves the above enumerated problems, and that exhibits excellent electrochemical properties such as high ionic conductivity and has flexibility and hence excellent workability.
Another object of the present invention is to provide a molded electrode that permits the construction of an electrochemical device having excellent operating characteristics, and that has excellent formability and workability.
Still another object of the present invention is to provide an electrochemical device that shows stable operation by resolving the problems associated with the volumetric change of the electrode active material occurring during the operation of the electrochemical device.